Fibers comprising a thermoplastic resin typical examples of which are polyethylene, polypropylene, and polyester have application in various fields since the fibers are easy to handle and can be processed at a comparatively low cost. Since these thermoplastic resins are hydrophobic (water absorption is lower than 1%), the fibers produced from such a resin are ready to be charged with static electricity and tend to produce various troubles when processed, for example, into non-woven fabrics. For the purpose of preventing the fibers from being charged with static electricity, or for the purpose of imparting hydrophilic property to the fibers when they are used for application, for example, in tea packs and filters, wherein fibers are required to have a hydrophilic property, such treatments as applying one of various kind of finishing agents containing a surface active agent on the surface of the fibers are being performed to adhere the finishing agent to the fiber.
In order to use such fibers to which a surface active agent is adhered in the field of foods, the surface active agent should be low in toxicity because the surface active agent will flows out when the fibers are put in water once or twice. Besides, such problems are raised that water permeability decreases and filtration resistance increases for tea packs and filters, respectively, since the hydrophilic property of the fibers is suddenly decreased due to the outflow of the surface active agent.
As the surface active agents which are small in the effluence from the fibers and can durably maintain the hydrophilic property of the fibers, sorbitan ester-ethylene oxide adducts are known. However, the sorbitan ester-ethylene oxide adducts have such a defect that the water in which the fibers or cloths having the adduct applied thereon were immersed are ready to foam or become cloudy since the adducts have a high foaming property.